Method and apparatus for examining defects in or on sheet material

ABSTRACT

For examining defects in sheet material, in particular bank notes, the sheet material is convexly curved and tested in the area of the convex curvature. A detector is disposed tangentially to an apex line of the convex curvature for detecting elevations on the bank note surface due to bank note defects against a light background. A suitable optic is used to image this silhouette onto the detector. The detector is formed as a pixel array, and the number and height of shaded pixels are assessed as measures of defect density and size and nature of defects.

[0001] This invention relates to a method and apparatus for examiningdefects in or on sheet material, in particular bank notes, in particularfor determining creases, tears, holes or dog-ears. The invention relatesin addition to a bank note processing machine having such an apparatus.

[0002] The main area of use for the invention is to determine defects inbank notes. However, the invention is suitable for examining any sheetmaterial, in particular for examining papers of value, whose quality cansink below a given standard through signs of wear.

[0003] Bank notes in circulation are generally tested for quality andauthenticity after returning to a commercial and/or national bank. Thistest is normally done automatically in specially developed bank noteprocessing machines. In case of a negative test result, the particularbank note is withdrawn from circulation. Quality is assessed withreference to so-called fitness criteria, which are determined forexample with reference to soiling, tears, creases, holes, dog-earsand/or stiffness of the tested note in comparison to a new note.

[0004] U.S. Pat. No. 5,955,741 discloses a plurality of methods forassessing the fitness of bank notes with reference to their stiffness.Bank note paper contains long fibers that break through frequent use, sothat notes lose their initial stiffness in the course of time. Thisstructural change of the bank note paper is detected in order toindirectly infer the stiffness or derive a corresponding fitnesscriterion for the note. According to one of the methods proposedtherein, the optical transmission or reflection properties of the noteare detected. The note is thus irradiated with IR light (transmissionmeasurement) or UV light (reflection measurement). The more IR lightpasses through the note or the more reflected UV light is scattered bythe note surface, the poorer the quality of the note is to be rated.

[0005] The method proposed in U.S. Pat. No. 5,955,741 permits only arough test of bank note properties, however. Large-area detection ofreflected and transmitted radiation per-permits only statisticalstatements about defects in the paper. The contribution and size ofindividual defects is not determined.

[0006] The problem of the present invention is to propose an improvedmethod and apparatus for examining defects in or on sheet material.

[0007] This problem is solved by a method and apparatus having thefeatures of the independent claims. The dependent claims relate toadvantageous developments and embodiments of the invention.

[0008] According to the invention, the sheet material is convexly curvedand the defects located in the area of the convex curvature detected.Convex curvature of the bank note makes any defects more evident. Brokenfiber ends protrude out of the paper, tears and holes are extended.Defects can thus be detected more easily.

[0009] Defects are preferably detected along an apex line or atindividual points of an apex line. However, the inventive solution alsoquite generally provides convex curvatures of sheet material that haveno apex line but a summit. Defects are accordingly detected in the areaof said summit. Defects are preferably detected by means of an opticalsensor. Optical sensors are inexpensive and available in numerousvariants, so that they can be integrated into existing bank noteprocessing machines at no great cost.

[0010] To permit the size and contribution of single defects to beindividually detected, a preferred embodiment of the invention providesthat an optical detector is disposed in an apex line plane of theconvexly curved sheet material and directed toward the apex line so thatthe apex line of the convexly curved sheet material forms for thedetector a kind of horizon above which defects of the bank note rise insilhouette. An apex line plane within the meaning of the invention isthus a plane tangential to the convex curvature, and the apex linewithin the meaning of the invention is defined by the tangent linebetween convex curvature and apex line plane. The convexly curved areaof the sheet material will also be referred to as the apex in thefollowing.

[0011] To permit optimal detection of the silhouette arising from thedefects, it is advantageous if the apex of the curved sheet material isdisposed against a light background. A uniformly light, homogeneousbackground can be obtained by means of a fluorescent lamp, a brightlyilluminated surface, an LED row, or an LED array with a scatteringmedium disposed therebefore.

[0012] Precision of the test results can be improved if an optic isprovided between the apex and the detector for imaging at least onepoint on the apex or apex line onto the detector. The imaging optic usedmay be for example a spherical, aspherical or cylindrical convergentlens or a self-focusing lens array (so-called Selfoc lenses).

[0013] A preferred detector includes a pixel array aligned parallel tothe apex line. This permits adjacent areas of the apex to be separatelydetected and evaluated. The pixel array is preferably formed as atwo-dimensional pixel array with pixels disposed in a uniform grid or asa one-dimensional pixel array with elongate pixels disposedperpendicular to the apex line. The individual pixels are formed asphotosensitive elements, preferably as photodiodes or charge-coupleddetector elements, so-called CCDs.

[0014] The silhouette caused by the defects is imaged on the pixels ofthe detector directed toward the apex line as a shadow, in particularagainst a light background when using an imaging optic. The more defectsare present in the sheet material, the more elevations the silhouettehas and accordingly more adjacent pixels of the detector are located inthe shadow. The larger the defects are, the higher the silhouette is inthe corresponding apex area and the more pixels disposed one above theother are located in the shadow. In the case of a one-dimensionaldetector array with elongate pixels disposed perpendicular to the apexline, the voltage per pixel is dependent on the height of the shadowfalling on the pixel. This permits a measure of local density of defectsto be derived from the number of unilluminated pixels, and a measure ofsize and/or an indication of the nature of the defects from the heightof unilluminated pixels. For this purpose an accordingly formedevaluation device connected with the detector is provided.

[0015] A specific preferred embodiment of an apparatus for carrying outthe test method provides that the convex curvature of sheet material iseffected on a convexly curved component. This can preferably be astationary linear element or a transport cylinder. Such components arereadily present in bank note processing machines or can be added withoutany great effort.

[0016] Additionally, belts can be provided to ensure that sheet materialrests reliably on the curvature of the convexly curved component. Thisreduces the danger of the apex line of convexly curved sheet materialmoving out of the focusing line.

[0017] Examination of sheet material can be effected during sheettransport so that the detector detects a silhouette changing in timewhich is evaluated by the evaluation device in real time. If defectsfail to meet a given fitness criterion according to number and/orheight, the corresponding bank note is withdrawn from circulation.

[0018] Apart from defects, the above-described system can also be usedto detect and evaluate light reflexes due to strongly reflective areas,such as security threads, adhesive strips, kinegrams, etc., so thatauthenticity features of sheet material can be tested in addition or asan alternative to quality.

[0019] In the following, the invention will be explained by way ofexample with reference to the accompanying drawings, in which:

[0020]FIG. 1 shows a schematic side view of a preferred embodiment ofthe invention in a side view;

[0021]FIG. 2 shows the apparatus from FIG. 1 schematically in a planview;

[0022]FIG. 3 shows a two-dimensional pixel array used according to theinvention;

[0023]FIG. 4 shows a one-dimensional pixel array used according to theinvention; and

[0024]FIG. 5 shows an apparatus according to a further preferredembodiment of the invention.

[0025]FIG. 1 shows schematically a side view of an apparatus forexamining defects on a bank note according to a first preferredembodiment. Note BN is transported over transport cylinder 10 rotatingin the direction of the arrow. The transport direction of note BN islikewise indicated by an arrow. Convex curvature of note BN on transportcylinder 10 causes individual defects 1 to emerge from the surface ofthe note in curvature area or apex 11. Defects 1 are showndisproportionately large for clarity's sake. They are normally smalldefects, for example ends of broken fibers standing out of the bank notepaper or the like. However, tears, holes and dog-ears also appear aselevations above the surface of note BN in the curvature area. FIG. 2shows the apparatus from FIG. 1 in a plan view. One can see that defects1 are fold 2, tear 3 and other defects 4 such as elevations, holes,protruding fibers, etc. In addition, reflective element 5, for example akinegram, is located on the surface of note BN.

[0026] A sensor is used to examine the note for defects 1 to 4 andreflective element 5. The sensor includes detector 20 directed towardapex 11 of transport cylinder 10 or note BN and located in apex lineplane 23. Detector 20 looks beyond apex 11, so to speak, so that apexline 12 forms a kind of horizon for detector 20. Defects 1 to 4 riseabove this horizon in silhouette. To image the silhouette optically ontodetector 20, optic 21, formed as a conventional spherical convergentlens here, is disposed between apex line 12 and detector 20. Dependingon the case of application, optic 21 can also consist of aspherical orcylindrical lenses. In addition to detector 20 and imaging optic 21, thesensor for examining defects 1 to 4 and reflective element 5 includes ahomogeneous, light background in prolongation of the optical axisleading from detector 20 to apex 11. The homogeneous light background isformed here by illuminating means 92, in particular a fluorescent tube.However, this may also be an illuminated light surface or, as seen inFIG. 5, LED row 26 or an LED array preceded by diffusing disk 27. Thismakes defects 1 to 4 appear to detector 20 as dark elevations above apexline 12 of apex 11 against the light background.

[0027] Detector 20 includes a pixel array. It may be two-dimensionalpixel array 24 with uniformly disposed, square pixels, as seen in FIG.3. However, it may also be one-dimensional pixel array 25 with elongatepixels oriented perpendicular to apex line 12, as seen in FIG. 4.Differently structured pixel arrays can of course also be used.

[0028]FIGS. 3 and 4 show the shadow of apex 11 of convexly curved noteBN cast on detector 20. Height 12′ marks the horizon or apex line 12.Below height 12′ all pixels are located in the shadow of apex 11.Insofar as pixels are located in the shade above height 12′, suchshadows are due to elevations or defects 1 to 4 rising above the notesurface. In FIG. 3, pixel array areas are designated a to d where theshadow goes beyond apex line height 12′. The silhouette in shadow area ais due to lateral tear 3 of note BN. The silhouette in shadow area b isdue to fold 2, which is already located behind apex line 12, asindicated by FIG. 2. Use of optic 21 causes the silhouette in area b tobe blurred, so that it can be filtered out with a suitable evaluationdevice. On the other hand, subsequent filtering out can be omitted if anoptic with long focal length is used, so that only shadows in the directapex line area are imaged onto the detector due to the low depth offocus. The silhouette in shadow area c may be due for example to fibersprotruding from the note, and the silhouette in shadow area d to anelongate hole or the like in the note.

[0029] The silhouette of the shadow changes constantly when note BN ismoved in the transport direction. Evaluation device 30 is used toevaluate the changing shadow patterns in real time. In the case oftwo-dimensional pixel array 24 shown in FIG. 3, each individual pixeldelivers a voltage that is between a lowest value in the case ofcomplete illumination and a highest value in the case of completeshading. Limiting values can also be used, so that a mainly illuminatedpixel does not deliver any voltage and a mainly shaded pixel delivers agiven voltage that is equal for all mainly shaded pixels. The number ofshaded pixels above apex line height 12′ serves as a measure of defectdensity in tested note BN. Furthermore, the silhouette height isevaluated with reference to the number of shaded pixels located oneabove the other. Silhouette height is assessed as a measure of the sizeof defects or as an indication of the nature of defects. In the case ofshadow area a, the exceptional silhouette height at the edge of the noteindicates for example that the note has a tear on the side.

[0030] Instead of two-dimensional pixel array 24, one-dimensional pixelarray 25 can also be used, as shown in FIG. 4. The voltage delivered bythe individual pixels depends on the height of their shading. The pixelon the extreme left thus delivers the highest voltage in the shownexample. In this case, defect density can also be inferred from thenumber of pixels delivering an elevated voltage, and defect size and/ornature of defects inferred from the voltage level of the individualpixels. The continuous measurement results in a temporal and thusthree-dimensional image of the note surface. This temporal aspect istaken into account upon evaluation and classification of the particularshadow patterns.

[0031] Strongly reflective areas of the note, which may be due tokinegram 5 for example, can also be detected by means of theabove-described apparatus since detector 20 receives an unusual amountof radiation for said areas so that the voltage delivered by theparticular pixels drops below the value of the background brightness. Anadditional detector can optionally be provided, which can be constitutedlike above-described detector 20 and in particular designed as a pixelarray, for detecting reflected light.

[0032]FIG. 5 shows a further special embodiment of the inventiveapparatus for examining defects in or on sheet material. The view ofFIG. 5 corresponds to the schematic view of the embodiment according toFIG. 1 with a few differences. Instead of fluorescent tube 22, LED row26 is provided in this case, whereby diffusing disk 27 disposedtherebefore converts the LED radiation into radiation homogeneous acrossthe surface. An especially homogeneous background can be obtained byusing an LED array wherein the individual LEDs are distributed across asurface. Here, too, further homogenization of the illumination can beobtained by a diffusing disk additionally disposed before the LED array.Instead of transport cylinder 10, convexly curved guiding plate 13 isprovided over which note BN is guided. Roller system 14 ensures thenecessary feed in the transport direction. Additionally a belt systemcan be provided in this embodiment, as in the embodiment shown in FIG.1, for urging note BN onto transport cylinder 10 or guiding plate 13 toreliably guide note BN The belts should of course be as narrow aspossible since they cover the surface of note BN under test so that thenote cannot be examined in the relevant area.

1. A method for examining defects (1 to 4) in or on sheet material (BN),in particular for bank notes, characterized in that the sheet material(BN) is convexly curved and defects (1 to 4) of the sheet material (BN)are examined in the area of the convex curvature (11).
 2. A methodaccording to claim 1, wherein at least one optical detector (20) is usedfor examining the defects (1 to 4).
 3. A method according to claim 2,wherein the optical detector (20) is directed toward an apex, inparticular an apex line (12), of the convex curvature (11) and disposedin the plane (23) of the apex or apex line (12).
 4. A method accordingto claim 3, wherein an optic (21) is provided between the apex or apexline (12) and the detector (20) for imaging at least one point in thearea of the apex or apex line (12) onto the detector (20).
 5. A methodaccording to claim 4, wherein a self-focusing lens or self-focusing lensarray is used as an optic (21).
 6. A method according to any of claims 3to 5, wherein the apex line (12) is disposed against a light background(22, 27) regarded from the detector (20).
 7. A method according to claim6, wherein a fluorescent lamp 22 serves as a light background.
 8. Amethod according to claim 6, wherein an illuminated surface serves as alight background.
 9. A method according to claim 6, wherein an LED row(26) or an LED array, in particular with a scattering medium (27)disposed therebefore, serves as a light background.
 10. A methodaccording to any of claims 2 to 9, wherein the detector (20) includes apixel array (24, 25).
 11. A method according to claim 10, wherein thepixel array (25) is formed as a one-dimensional pixel array withelongate pixels, or a two-dimensional pixel array with square pixels.12. A method according to claim 10 or 11, wherein the number ofunilluminated pixels is assessed as a measure of local density ofdefects (1 to 4).
 13. A method according to any of claims 10 to 12,wherein the height of unilluminated pixels is assessed as a measure ofsize and/or an indication of nature of defects (1 to 4).
 14. A methodaccording to any of claims 1 to 13, wherein convex curvature is effectedon a convexly curved component (10, 13).
 15. A method according to claim14, wherein convex curvature is effected on a transport cylinder (10).16. A method according to claim 14 or 15, wherein the sheet material(BN) is urged against the convexly curved component (10, 13) by means ofone or more belts.
 17. A method according to any of claims 1 to 16,wherein convex curvature and examination of defects (1 to 4) areeffected with the sheet material (BN) moving.
 18. A method according toany of claims 1 to 17, wherein light reflexes due to strongly reflectiveareas (5) of the sheet material (BN) are detected and evaluated.
 19. Anapparatus for examining defects (1 to 4) in or on sheet material (BN),in particular for bank notes, characterized by a device (10, 13) forconvexly curving the sheet material (BN) and at least one detector (20)for detecting defects (1 to 4) in the area of the convex curvature (11).20. An apparatus according to claim 19, wherein the detector (20) is anoptical detector.
 21. An apparatus according to claim 20, wherein theoptical detector (20) is directed toward an apex, in particular an apexline (12), of the convex curvature (11) and disposed in the plane (23)of the apex or apex line (12).
 22. An apparatus according to claim 21,wherein an optic (21) is provided between the apex or apex line (12) andthe detector (20) for imaging at least one point in the area of the apexor apex line (12) onto the detector (20).
 23. An apparatus according toclaim 22, wherein the optic (21) is a self-focusing lens orself-focusing lens array.
 24. An apparatus according to any of claims 21to 23, wherein the apex or apex line (12) is located against a lightbackground (22, 27) regarded from the detector (20).
 25. An apparatusaccording to claim 24, wherein a fluorescent lamp (22) serves as a lightbackground.
 26. An apparatus according to claim 24, wherein anilluminated surface serves as a light background.
 27. An apparatusaccording to claim 24, wherein an LED row (26) or an LED array, inparticular with a scattering medium (27) disposed therebefore, serves asa light background.
 28. An apparatus according to any of claims 20 to27, wherein the detector (20) includes a pixel array (24, 25).
 29. Anapparatus according to claim 28, wherein the pixel array (25) is formedas a one-dimensional pixel array with elongate pixels or, atwo-dimensional pixel array with square pixels.
 30. An apparatusaccording to claim 28 or 29, comprising an evaluation device (30) inwhich the number of unilluminated pixels is assessed as a measure oflocal density of defects (1 to 4).
 31. An apparatus according to any ofclaims 28 to 30, comprising an evaluation device (30) in which theheight of unilluminated pixels is assessed as a measure of size and/oran indication of nature of defects (1 to 4).
 32. An apparatus accordingto any of claims 19 to 31, wherein the device (10,13) for convexlycurving the sheet material (BN) is a curved component.
 33. An apparatusaccording to claim 32, wherein the device for convexly curving the sheetmaterial (BN) is a transport cylinder (10).
 34. An apparatus accordingto claim 32 or 33, wherein belts are provided which can urge the sheetmaterial (BN) against the convexly curved component (10, 13).
 35. Anapparatus according to any of claims 19 to 34, which is formed forexamining the sheet material (BN) with the sheet material (BN) moving.36. An apparatus according to any of claims 19 to 35, wherein thedetector (20) and/or an additional detector is formed or provided fordetecting light reflexes due to strongly reflective areas of the sheetmaterial (BN).
 37. An apparatus according to claim 36, wherein theadditional detector is formed as a pixel array.
 38. A bank noteprocessing apparatus comprising an apparatus according to any of claims19 to 37.